In an induction motor for a vehicle, at the time of the operation the temperatures of parts of the motor are raised by a copper loss caused by current flowing through a stator coil and a rotor conductor and an iron loss developed in a stator core and a rotor core by a rotating magnetic field. For this reason, there has been conventionally used a structure in which cooling air is forcibly taken into the motor from the outside by a fan fixed to a rotary shaft to cool the interior of the frame of the motor (see, for example, patent document 1).
When a self-cooled induction motor as described above is used in a heavy snowfall area, cooling air to the motor is sucked in proportion to the number of revolutions of the motor even at the time of startup and at the time of low load and hence snow and water droplets are brought into the motor together with the cooling air, thereby bringing about a state in which the snow and the water droplets prevent the revolution of the motor and are again frozen to cause damage to the insulating surface and parts of the motor after the motor is stopped.
When the self-cooled induction motor as described above is used, a plurality of rotor conductors cut through wind to develop a sound. For example, when the number of rotor conductors is forty six and the rotor rotates at a rotational speed of 3000 rpm, the rotor conductors cut through the wind to steadily develop a sound of a frequency of 2300 Hz (=50×46) and a fan cuts through the wind to develop a sound similarly to diffuse the sounds to the outside, so the induction motor develops a very large harsh sound.
[Patent document 1] JP-A-2001-45711